The Relationship between Heart Rate Variability and Adiposity Differs for Central and Overall Adiposity

Abstract

While frank obesity is associated with reduced HRV, indicative of poorer autonomic nervous system (ANS) function, the association between body mass index (BMI) and HRV is less clear. We hypothesized that effects of adiposity on ANS are mostly mediated by visceral fat and less by subcutaneous fat; therefore, centrally distributed adipose tissue, that is, waist circumference (WC), should be more strongly associated with HRV than overall adiposity (BMI). To examine this hypothesis, we used data collected in a subset of the Baltimore Longitudinal Study of Aging to compare strength of association between HRV and WC to that of HRV and BMI. Time domain HRV variables SDNN (standard deviation of successive differences in normal-to-normal (N-N) intervals) and RMSSD (root mean square of successive differences in N-N intervals) were calculated from 24-hour Holter recordings in 159 participants (29-96 years). Increasing WC was associated with decreasing SDNN and RMSSD in younger but not older participants (P value for WC-by-age interaction = 0.003). BMI was not associated with either SDNN or RMSSD at any age. In conclusion, central adiposity may contribute to sympathetic and parasympathetic ANS declines early in life.

Figure 1

Histograms of heart rate variability variables with a smooth kernel density estimate (dashed line) and gamma distribution (solid line). RMSSD: root mean square of successive differences; SDNN: standard deviation of N-N intervals.

Figure 2

Data points are observed data; lines represent the predicted value of nighttime RMSSD (top row) and daytime SDNN (bottom row) (if the adiposity measure increased for an average participant at two ages: 45 years (solid lines) and 85 years (dashed lines)). Standardized waist circumference and standardized BMI are shown on x-axes to scale the measures similarly. The figures show differing relationships between waist circumference and HRV measures at young and old ages but not between BMI and HRV measures. Negative relationships were observed between waist circumference and RMSSD and SDNN in the 45 yo but no significant relationship in the 85 yo. Comparatively, the SDNN-BMI relationship is attenuated, compared to WC, for the 45-year olds and the RMSSD-BMI relationship is relatively flat at 45 and 85 years, with neither being statistically significant. *x-axes are standard deviations from mean waist circumference (women: 88 cm, men: 98 cm) or BMI (27 kg/m²). *Graded gray-scale data points from population illustrate increasing age (lighter is younger, darker is older, 26–96 y). Models were adjusted for sex, race, OGTT glucose, blood pressure, and the interactions age-by-sex, age-by-race, the sex-by-OGTT glucose. OGTT glucose = 2-hour oral glucose tolerance test glucose result; RMSSD:root mean square of successive differences; SDNN:standard deviation of N-N intervals.